(1) Field of the Invention
This invention relates to an improvement in the purification process of an aqueous acrylamide solution which has been obtained by catalytically hydrating acrylonitrile with water in the presence of a copper-containing catalyst. The invention particularly relates to a process for the preparation of aqueous acrylamide solutions which are useful as the starting material of high molecular weight flocculant.
(2) Description of the Prior Arts
Acrylamide is valuable as the starting material of acrylamide polymer which has versatile utilities such as paper-reinforcing agent and high molecular weight flocculant. The preparation of acrylamide has become relatively simple, as the conventional sulfuric acid process is replaced by catalytic hydration of acrylonitrile in the presence of a catalyst composed chiefly of copper metal. According to the latter process, the acrylamide is obtained as an aqueous solution, and can be advantageously fed directly into the polymerization reaction system. When the aqueous acrylamide solutions obtained upon such hydration are immediately subjected to polymerization, however, acrylamide polymers having high performance of flocculation and solubility cannot be obtained. Not only that, in extreme cases the polymerization reaction itself fails to progress.
Presumably such phenomena are caused by the various impurities present in the aqueous acrylamide solution in minor amounts due to the method of preparing said solution, which adversely affect the polymerization reaction. Such impurities would include unreacted acrylonitrile, metal ions such as of copper eluted from the catalyst component, the impurities in the starting acrylonitrile and the side products such as organic acid. Of the above-named, unreacted acrylonitrile can be easily removed by conventional means such as distillation. Also the metal ions such as of copper can be easily removed by a treatment with cation exchange resin. Even after the distillation for removing unreacted acrylonitrile and the cation exchange resin treatment, the aqueous acrylamide solution cannot serve as a satisfactory starting material for a high molecular weight flocculant having a high performance of flocculation and water-solubility, although it may be satisfactory for making a paper-reinforcing agent.
That is, when acrylamide is used as a paper-reinforcing agent, acrylamide polymers of relatively low molecular weight, such as in the order of several ten thousands, can be used. When it is to be used as a high molecular weight flocculant, normally high molecular weights such as several millions are required. Recently acrylamide polymers of super high molecular weight such as above ten millions are manufactured for this purpose. Furthermore, with the increase in variety and degree of complication of industrial waste waters, various types of high molecular weight flocculants are produced, i.e., not only their molecular weights are drastically increased, but some are imparted with anionic property through the hydrolysis of amide groups in the polymers, or others are caused to contain monomers other than acrylamide through copolymerization.
Accompanying such increased versatility and improved performance of high molecular weight flocculants, the starting acrylamide also is required to have improved quality, and hence the heretofore permitted very minor amounts of impurities must now be removed.
As the means to remove impurities from the aqueous acrylamide solutions prepared by conventional sulfuric acid process, their treatment first with cation exchange resins and then with anion exchange resins was proposed in J. Appl. Chem. of U.S.S.R. vol. 41, p. 820 (1968). Also concerning the aqueous acrylamide solutions obtained by the catalytic hydration, a purification method of treating them with OH- or weak acid salt-form, strongly basic anion exchange resin is disclosed in Japanese Laid-Open Patent Application, Publication No. 82011/75. Furthermore, the process of treating the latter solutions with a mixed bed composed of a cation exchange resin and an anion exchange resin is known from Japanese Laid-Open Patent Application, Publication No. 83323/75.
As has been disclosed, for example, in Japanese Laid-Open Patent Applications, Publication Nos. 66618/79, 73727/79 and 74890/79, however, ordinary ion-exchange resin treatments fail to remove certain impurities depending on their structures. In order to cope with the shortcoming, the named prior arts proposed respectively to add ammonia, urea or the like to the catalytic hydration reaction system, to add to said system a meta-substituted phenolic compound, or to apply a unique polymerizing method to the aqueous acrylamide solutions.
Our research works also proved that the aqueous acrylamide solutions treated with an OH-form, strongly basic anion exchange resin, or with a weak acid salt form strongly basic anion exchange resin, or with the aforesaid mixed bed, failed to polymerize under certain polymerization conditions, or if polymerized, failed to serve as the starting material of a high molecular weight flocculant having excellent performance of flocculation and solubility in water. A number of causes may be enumerated for such phenomena. Our studies disclosed that, the bifunctional vinyl monomers such as methylene bis-acrylamide, which are present in acrylamide as the impurities, are incorporated into the polymer chain of the polyacrylamide during the polymerization reaction, rendering the polymer to have a branched structure, not the straight chain. Such a branched polymer is less effective than the straight chain polymer as a high molecular weight flocculant. Hence, it must be added to the waste water to be treated by an increased amount. Not only that, the turbidity of the treated waste water cannot be sufficiently improved. Thus the flocculant fails to achieve satisfactory results. If the molecular weight of the acrylamide polymer is increased in an attempt to reduce its amount of addition to the waste water, the polymer having such a branched structure shows poor solubility, failing to function as a flocculant.